Absorption bands and extinction in small interstellar particles

The production of discrete line and broad-band extinction by small interstellar oxide and silicate particles is discussed quantitatively. Restrictions on particle size and refractive index that are required to produce ‘pure’ absorption features are reviewed. The relationship between optical depth in interstellar extinction and absorption coefficients for bulk materials is used to reach some general conclusions concerning the diffuse interstellar features, VUV extinction and the composition of interstellar dust. It is noted that charge transfer bands of ions such as Fe³⁺ may be detectable in the VUV spectrum of dust. Several effects that lead to the enhancement of oscillator strength by 10³–10⁴ in small particles are discussed.